This project will test three specific hypotheses that in tracheal smooth muscle: I. the long functioning range of length results from plastic rearrangements of the filament array; II. the slowing of velocity during sustained contraction is due to thick filament lengthening; and III. the activation of contraction is regulated by a second process in addition to myosin phosphorylation. Hypothesis I derives from the preliminary finding that over a 3-fold range of length the muscle has a 2.3-fold change in shortening velocity, a 1.9- fold change in compliance, with less than 25% change in isometric force. These findings indicate that the number of contractile units in series varies directly and almost in proportion to the overall muscle length. The findings by others that thick filaments can be evanescent suggests that this plasticity in structure may result from the dissolution and reformation of thick filaments. This consideration leads to hypothesis II, that the lengthening of thick filaments slows velocity. The additional findings by others that phosphorylation of myosin, which is required for activation, causes folded, inactive myosin to unfold in a way that allows it to form thick filaments suggests that the primary role of myosin phosphorylation may be to control the structural changes. This consideration leads to hypothesis II, because there must be an additional mechanism that will distinguish phosphorylated myosin in thick filaments from phosphorylated myosin free in the cytoplasm. The three hypotheses will be tested by measuring and controlling length of the central segment of the muscle, and using the mechanical responses of the central segment (its compliance, shortening velocity, and transient tension responses to sudden length changes) to indicate changes in the number of contractile units in series and parallel. In studies of activation the instantaneous maximum power will be used to signal changes in the number of activated myosin crossbridges, which will be compared with changes in myosin phosphorylation, and tension transients will be used to signal changes in the number and arrangement of attached crossbridges. Length perturbations of the fully activated muscle will be used to detach crossbridges and reduce isometric force. The time course of redevelopment of force, stiffness, and power will be compared with the rate of rise of the same parameters at the onset of stimulation to distinguish activation processes that occur early in contraction. The results will provide new insights into the contractile mechanisms of smooth muscle. These insights will further our understanding of patho-physiological processes in diseases, such asthma and hypertension, where muscle tone is increased.